I have 1 amazing best friend, but how can I move on and make more friends?

Include your 1 best. Friend in the process. Every best friend is different and a person doesn't have to give one up to have another. Give the current best friend some credit for being a friend and understanding the need for more than one friend. You might approach the subject with that " best friend" and be pleasantly surprised with agreement. If that isn't what happens then best friend may not be " best friend".
Friends. You are very fortunate to have a best friend. A great many people do not have a best friend. Why would you need more best friends?